Technological advances over the last decades have increased the use of electronic devices in many areas of everyday life. The space around people, almost in any area of the world, is immersed in a variety of emitted electro-magnetic waves. These electro-magnetic (EM) emissions vary in power and frequency. Some devices, such as cell phones, claim the spectrum as part of their intended operation. Some devices, like computers and displays, radiate unwanted emissions that can compromise the operation of other electronic devices. These EM emissions can be particularly harmful if they cause malfunctions of devices, such as pacemakers, and airplane navigation or instrument landing equipment.
In the United States, the Federal Communications Commission (FCC) regulates the amount of radiated emissions according to the class of a device. Class A targets industrial environments and class B targets residential environments. Class A devices have a more relaxed specification than class B. Another class, for open-box equipment, has slightly less stringent specifications than the permitted emissions from packaged products.
Electro-magnetic interference (EMI) requirements are often overlooked by product engineering at the specification phase, and are often an afterthought when the equipment does not meet the specification. This can lead to very expensive last moment shielding or re-design, which is clearly undesirable. Likewise, many designs rely on over-designed and heavy shielding, which is not a viable option in many devices, such as open-box equipment.
The increased data storage and fast access to data in the modern information technology world has increased the demand for high-speed data transfer. There are numerous reasons why the data needs to be transferred at high speed between the integrated chips on one electronic printed circuit board (PCB), between the PCBs within a specific piece of electronic equipment, or even between different pieces of electronic equipment. The data transfer can occur over different media, such as optical fiber or copper wire, or wirelessly. Optical fiber has distinct advantages of a large throughput and no significant EMI to other electronic equipment. But the price for such data transmission devices, and fiber itself, is still quite high. Wireless means of data transfer are very popular, and have many advantages, but the transmission suffers from smaller bandwidth than fiber or wire-bound transmission. Equipment manufacturers prefer to use wire-bound data transfer because it is cheaper than optical data transfer components and wireless components, and offers reasonable bit error rates (BER) and high information capacity.
The use of high-speed data links is replacing data transfer over a bus where a number of slower speed digital signals were used. The use of a bus is acceptable for fast data transfer within a PCB, but between different PCBs it is much more suitable to use high-speed links, because a bus requires connectors with large number of pins to carry a large number of signals. Also, a large number of signals cannot usually be sent differentially and sending signals single-ended usually causes more radiation or more signal distortion. Typical high-speed interfaces in use today include high speed Universal Serial Bus (USB), Fiber-channel, Infiniband, SATA, SAS, and Gigabit Ethernet. These high-speed interfaces start from half a Gigabit per second (Gb/sec) and are now offering more than 6 Gb/sec data rates, with the future information rates of over the 10 Gb/s on single high-speed input/output (HSIO) devices expected. Each of these interfaces uses differential signal lines to carry high-speed digital data.
Signals are generated at a transmit input/output (IO) and pass through the PCB on which IO is residing to some kind of connector. The connector enables the signals to be carried over cables or the backplane to another PCB that contains the receiver IO. Differential signaling has several advantages over single ended signaling. Although the differential signal uses two conductors to convey the signal from the transmitter to the receiver, the signal at the receiver is more immune to the various noise sources, as the noise sources affect both conducting lines that carry the signal in a similar fashion. Thus, the difference between the signal waveforms does not contain the effects of the noise sources. Sometimes the output signals can be deterministic; this means that there is no message that needs to be sent over the lines. This is the case when a clock or some sort of pseudo-random sequence (PRBS) is sent in order to keep some circuits working. The EMI specifications need to be satisfied for these signals as well.
FIG. 1 shows why transmission lines that carry only differential signals have reduced EM radiation. The system of FIG. 1 includes an IO driver 40, a differential transmission line 42 and a termination load 44. Ampere's Law specifies the total current enclosed by the line integral of magnetic intensity vector H over some closed contour.
      I    total    =                    ∮                  H          ⇀                    Closed_Contour        ·          ⅆ              l        ⇀            So, in case of a differential signal, where i1=−i2, the total current Itotal is equal to zero, and the magnetic intensity H is equal to zero. Hence, in an ideal differential transmission line there are no EM emissions. If the two signal paths following each other closely and the net sum of current in the two conductors is zero, then there is no radiation.
However, differential signal lines also carry a common mode signal. The common mode signal is referenced to ground and the ground does not follow the signal path as closely as the two signal paths that carry the differential signal follow each other. This is shown in FIG. 2. The common mode current loop 46, which has a return path to ground, causes undesirable EMI if i1< >i2. The larger the physical size of the loop 46, the more radiation can be expected.
The common mode signal on the two differential lines is defined as the average of the two signals. If the two differential signals are biased and above the DC, which usually is the case if the high-speed transmitter uses only positive power supply voltage, the common mode signal has a DC component. The DC component can be easily blocked by using series capacitors, so many high speed data transmission interface standards specify use of series blocking capacitors. However, higher frequencies common modes need to be blocked as close as possible to the source to minimize the size of the common mode loop. The common mode can result in radiation at discrete frequencies related to multiples of the symbol rate. These discrete frequencies are especially harmful and they are the usual reason a device does not meeting the EMI requirements. Therefore, some means of reducing or filtering the common mode is required.
One way to reduce the common mode radiation is to modulate the clock that clocks out the data with low frequency modulation using spread spectrum clocking (SSC). This spreads the spectrum of the clock, while at the same time spreading the discrete frequencies due to the common mode. A problem with this kind of EMI reduction is that modulation has to be relatively wide, and, although the receive clock recovery phase lock loop (PLL) can handle it, there can be a problem with first-in-first-out (FIFO) under/over-flow.
Another way to reduce common mode radiation is to filter the common mode. Care must be taken not to distort the differential signal. The most common way of implementing common mode filtering is use of the common mode chokes. Known approaches use ferrite based common mode chokes. The common mode chokes, based on a high level of magnetic coupling, present a high impedance for common mode and they are broad-band. However, they cannot achieve more than 10 to 15 dB of attenuation in the frequency range of interest without seriously affecting the differential mode. Sometimes this is not enough and in some cases engineers try to use two common mode chokes in series to improve the performance. FIG. 3 shows prior art common mode attenuation using a common mode choke, the operation of which can be described with reference to the following equations:
            V      ⁢                          ⁢      1        =                  L        ⁢                                  ⁢                  1          ·                                                    ⅆ                I                            ⁢                                                          ⁢              1                                      ⅆ              t                                          +              M        ·                                            ⅆ              I                        ⁢                                                  ⁢            2                                ⅆ            t                                          V      ⁢                          ⁢      2        =                  M        ·                                            ⅆ              I                        ⁢                                                  ⁢            1                                ⅆ            t                              +              L        ⁢                                  ⁢                  2          ·                                                    ⅆ                I                            ⁢                                                          ⁢              2                                      ⅆ              t                                          where for L=L1=L2M=k·√{square root over (L1·L2)}=k·L We get:
            V      ⁢                          ⁢      1        =                  (                  1          ±          k                )            ·      L      ·                                    ⅆ            I                    ⁢                                          ⁢          1                          ⅆ          t                                V      ⁢                          ⁢      2        =                  (                  1          ±          k                )            ·      L      ·                                    ⅆ            I                    ⁢                                          ⁢          2                          ⅆ          t                    For differential signals I1=−I2, k is negative. For extremely tight coupling k˜1, therefore there is virtually no voltage drop over the common mode choke for a differential signal, but for a common mode signal and tight coupling the voltage drop is proportional to (1+k)=˜2. This means that the common mode is attenuated. If the coupling is not very tight, then not only is the common mode attenuation not as high, but the differential mode is attenuated. For high-speed data communication, with data rates in the range of 10 GHz, the common mode chokes are typically made of bifilar windings wound around the ferrite bead. For this type of common mode choke, the coupling factor is in the range of 0.7 to 0.9, which leads to substantial attenuation of differential mode signals. This also means that it is preferable to use the tight magnetic coupling, which requires a fairly large area if implemented on chip or package. It is also challenging to achieve magnetic coupling factors of even 0.8. The ferrite based chokes need to be mounted on the top of the PCB and this requires PCB vias that can be detrimental to performance at high frequencies. If the differential lines are on top of the PCB than they can radiate. Also, the mounting pads for the common mode chokes will present discontinuity.
Having magnetic coupling of approximately 0.8 can also limit the value of the self inductances of high speed data common mode chokes, otherwise the attenuation of the differential mode will be too high. Therefore, as the values of self-inductances of common mode chokes generally need to be small, the common mode attenuation, which is proportional to the self-inductance, is also not large enough. Also, the common mode chokes do attenuate common mode proportional to frequency, so higher the frequency the higher the attenuation of the common mode. The spectrum of the common mode contains the discrete components and it is advantageous if we can build a structure that can attack and filter specific frequencies.
Therefore, it is desirable to have a structure that can perform common mode filtering well without undue attenuation of the differential signal, and preferably that does not require tight or high magnetic coupling for its operation.